Plants use light energy between 400 and 700 nanometers, the region known as Photosynthetically Active Radiation or PAR.Illumination for plants, also known as "irradiance", is sometimes measured in PAR watts per square meter (W/m2). Another means of measuring light quantity for plant growth involves discrete units of quantum flux in the PAR region called "photons". Photon flux is commonly measured in units of micromoles per square meter per second (µmoles/m2/s), where 1 mole of photons = 6.022 x 1023 photons.This is an objective measure since it directly indicates how much light energy is available for plants to use in photosynthesis. However, lamp manufacturers typically rate their lamps in lumens, a measure of light in the spectrum visible to humans. Moreover, most lighting engineers measure lighting levels in lumens per square meter (lux) or per square foot (foot-candles). Since the spectral sensitivities of plants and humans are quite different, there is no direct method of converting the units without evaluating the full range of spectral characteristics for a given light source.The calculator and table below use approximate conversion values for radiation of 400-700 nm from different lamp types, taken from the Plant Growth Chamber Handbook, 1997. Actual values may depend upon luminaire, lamp, ballast, and hours of use.

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Photon values are in µmoles/m2/s. For other conversions, divide lux by 10.764 to obtain foot candles, or multiply foot candles times 0.0929 to obtain lux.